As a joining material used for mounting electronic components, solder (solder paste) is widely used.
For Sn—Pb-based solder which has been widely used, methods of bonding with temperature hierarchy are widely applied in which high-temperature solder, for example Pb rich Pb-5Sn (melting point: 314 to 310° C.) or Pb-10Sn (melting point: 302 to 275° C.) is used to solder at a temperature of 330 to 350° C., following by using low-temperature solder, for example an Sn-37Pb eutectic crystal (183° C.) to solder at a temperature equal to or lower than the melting point of the high-temperature solder described above, whereby bonding is established by soldering without melting the high-temperature solder used in the preceding soldering.
Such bonding with temperature hierarchy is applied in, for example, a type of semiconductor devices in which a chip is die-bonded and semiconductor devices for flip-chip bonding, and is an important technique which is used in such a case where bonding is established in a semiconductor device, followed by further bonding the semiconductor device itself to a board by soldering.
As a solder paste for use in this application, for example, a solder paste comprising a mixture of (a) a second metal (or alloy) ball consisting of a second metal such as Cu, Al, Au and Ag or a high-melting point alloy containing those metals and (b) a first metal ball consisting of Sn or In has been proposed (see Patent Document 1).
Patent Document 1 also discloses a joining method using a solder paste and a method of producing electronic equipment.
When soldering using the solder paste of Patent Document 1, a solder paste containing low-melting point metal (e.g. Sn) balls 51, high-melting point metal (e.g. Cu) balls 52 and a flux 53 as schematically shown in FIG. 3 (a) is heated and thereby reacted and after soldering, a plurality of high-melting point metal balls 52 are connected together via an intermetallic compound 54 formed between a low-melting point metal derived from the low-melting point metal ball and a high-melting point metal derived from the high-melting point metal ball as shown in FIG. 3 (b), and an object to be joined is bonded/connected (soldered) by this connected body.
In the case of the solder paste of Patent document 1, however, a solder paste is heated in a soldering step to thereby generate an intermetallic compound of a high-melting point metal (e.g. Cu) and a low-melting point metal (e.g. Sn), but a combination of Cu (high-melting point metal) and Sn (low-melting point metal) has a low diffusion rate, so that Sn, a low-melting point metal, remains. A solder paste in which Sn remains may suffer a considerable reduction in bonding strength under a high temperature, and become unusable for some kinds of products to be joined. Furthermore, Sn remaining in the soldering step may be molten to run off in a subsequent soldering step, thus raising a problem of low reliability as high-temperature solder for use in bonding with temperature hierarchy.
That is, for example, if a semiconductor device is produced through a soldering step in a process of producing a semiconductor device, and thereafter the semiconductor device is mounted on a board by a method of reflow soldering, Sn remaining in the soldering step in the process of producing a semiconductor device may be molten to run off in the reflow soldering step.
For forming a low-melting point metal fully into an intermetallic compound so that Sn does not remain, heating at a high temperature and for a long time is required in the soldering step, but it is actually practically impossible in view of productivity.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-254194